1. Field of the Invention
The present invention relates to an index control apparatus for the tool rest of an NC lathe and, more particularly, to an improved index control apparatus for quickly driving the tool rest to a desired indexed position in accordance with an NC machining command.
2. Description of the Related Art
In an NC lathe, not only the main spindle on which a work is mounted but also the indexing operation of the tool rest on which a plurality of tools are mounted and a tool is selected among these as desired is numerically controlled, thereby enabling continuous unattended operation of the NC lathe for subsequently carrying out various complicated machining operations in accordance with a predetermined program.
In such an NC lathe, the index control of the tool rest is very important for selecting a plurality of desired tools for combining complicated machining operations. In addition, since the positioning accuracy exerts influence directly on the machining accuracy, a control apparatus which enables a quick indexing operation with a high swivelling velocity and a high positional accuracy is demanded.
Index control for a tool rest is divided into control of the tool rest swivelling operation and control of the clamping/ unclamping operation. The tool rest swivelling operation is generally carried out by an induction motor which is subjected to inverter control or by using a hydraulic pressure, and the clamping/unclamping operation is carried out by detecting the indexed surface of the tool rest by a positional sensor and by the operation of a hydraulic valve. A series of these swivelling and clamping/unclamping operations is sequentially controlled on the basis of an NC command.
FIG. 9 schematically shows a conventional index control apparatus. In order to obtain a desired indexed surface by swivelling a tool rest 10 in the direction indicated by the arrow, an induction motor 12 subjected to inverter control is used.
To the turret spindle 10a of the tool rest 10, the main shaft 12a of the induction motor 12 is connected through reduction gears 14 and 16 so that it is possible to swivel the tool rest 10 to a desired indexed position by the rotation of the induction motor 12.
Although the detailed structure is not shown in FIG. 9, the reduction gear 14, for example, of the reduction mechanism incorporates a clamping/unclamping mechanism which is driven by a hydraulic cylinder. The tool rest 10 is swivelled by the induction motor 12 in an unclamped state and when a predetermined indexed surface is obtained, the clamping operation is carried out by the hydraulic cylinder.
A general purpose inverter 18 is provided for driving the induction motor and a sequence control unit 24 is provided for supplying an inverter control signal to the general purpose inverter 18 on the basis of an index command supplied from an index commander 20 and a signal supplied from a position detector 22 such as a limit switch for detecting the position of the tool rest 10.
The sequence control unit 24 provides a start command ST, a direction command RV and a velocity command VC for the general purpose inverter 18. The position and the velocity of the induction motor 12 are controlled on the basis of these commands, whereby the indexed surface of the tool rest 10 is determined.
The index commander 20 includes a program memory 26 and a program interpreter 28. In the program memory 26, the index command for the tool rest 10 determined by an NC command is stored and the program interpreter 28 reads the index command stored in the program memory 26 and supplies a desired index command to the sequence control unit 24.
In this way, the index control apparatus for a tool rest can automatically and unattendedly control the selection of a tool on the basis of a desired NC program.
In the conventional apparatus, however, since the tool rest is swivelled by the inverter-controlled induction motor, it is impossible to set the accelerating or reducing velocity for swivelling the tool rest as desired, so that a comparatively long time is required for swivelling the tool rest and index adjustment, which leads to a long NC machining interval.
Especially, since the fine adjustment of the velocity is impossible, the swivelling velocity is approximately the same whether the index angle of the tool rest is small or large, so that the time required for indexing operation is very large with respect to a large swivelling angle.
When the velocity is adjusted by all means in the conventional apparatus, it is necessary to replace mechanical parts such as a reduction gear mechanism and, in practice, the inverter-controlled induction motor can hardly adjust the swivelling velocity and the accelerating or reducing velocity.
In addition, it is impossible to obtain a large holding torque under the control of the induction motor at the time of the suspension of the motor, so that the tool rest 10 sometimes deflects or deviates from the desired index position by a minute angle, thereby causing a clamping error, which exerts a serious influence on the machining accuracy of an NC lathe.